Although intercalation compounds have dominated commercial energy storage devices such as Li-ion batteries, it is still a major challenge to discover higher capacity materials with suitable cation sites, good redox potentials, and structural stability. Recently, two-dimensional (2D) materials (e.g., graphene, MoS2) have attracted attention due to their structural and electronic characteristics, with hybrid structures having interstratified structures of particular interest. The synthesis of these hybrid structures from nanosheet building blocks that can be reassembled, restacked, or deposited layer-by-layer, is an active field of research. Since conventional intercalation compounds are layered structures, they can in theory also be exfoliated into 2D nanosheets. Analogous hybrid structures can also be realized through the reassembly of these nanosheet components.
The strong interlayer bonding found in layered transition metal oxides makes the exfoliation process for these materials more difficult than in graphene and MoS2, which have weak interlayer van der Waals bonding. In order to exfoliate these materials, protonated forms of the metal oxide are typically prepared and then ion-exchanged in solutions containing tetraalkylammonium cations, which leads to exfoliation through the insertion of the bulky cations and osmotic swelling. For compounds relevant to electrochemical energy storage applications, however, this proton-exchange method is ineffective since it can lead to irreversible binding of protons or the formation of hydrated phases, which can have detrimental effects on electrochemical properties.